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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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400                                 ALASKAN GLACIER STUDIES
front of the glacier.   Witherspoon's map made in 1900 (Fig. 58) shows a normal convex glacier terminus.
Slight Advance in 1906 and 1906. A slight interruption of the balanced condition of the previous years took place in 1905 and 1906, for, (a) Webster Brown's photograph taken in August, 1905, shows a little crevassing of the hitherto stagnant moraine-covered northern margin; (b) Hegg's photograph of July 20,1906, shows jagged pinnacles of crevassed black ice in this northern portion, at a time when the middle of the glacier was fronted, at least in part, by a gravel bar; and (c) these photographs and the earliest maps by the railway engineers (Fig. 59) show the northern margin projecting eastward a short distance beyond the middle of the front, giving the terminus a concave shape, contrasting decidedly with the convex terminus as mapped by Hayes in 1891 and Wither-spoon in 1900.
That the 1905 and 1906 forward movement was slight is indicated by the maintenance of a fairly uniform average river width from 1900 to 1908, although it is possible that the width remained constant while the glacier gradually forced the river eastward. Even if this was the case, maps prove that the displacement was very slight, and of quite a different order from the 1910 advance.                                           /
Normal Condition from 1906 to 1909. From 1906 to 1909 the glacier seems to have resumed the balanced condition during which the steady supply of ice brought forward to the river was approximately equal to that taken away by melting and as icebergs, so that, while forward motion took place all the time in the glacier, its terminus neither advanced nor retreated appreciably.
The photographs by Webster Brown, in August, 1905, and by J. L. McPherson, in January, 1906, show that the middle of the front of Childs Glacier formed a nearly straight north-south line, while the northern margin projected Eastward very slightly beyond the middle, which was being cut back to a fairly uniform position by the river. This condition was maintained up to 1909. Sometimes the glacier and sometimes a gravel bar (PL CL, A) formed the western bank of the river, the condition varying with the time of year (as in 1898), due we believe, to the variations in river level and the position of its channel rather than to advance and retreat of the ice front. Thus a railway survey made early in the summer of 1905 indicates no gravel bar in front of the glacier, but Webster Brown's photograph shows that a narrow gravel bar was exposed in front of parts of the southern half of the ice front on August 16,1905; and in October, 1905, one of the railway engineers, Mr. Murchison, saw the whole face of Childs Glacier separated from the river by a gravel bar 40 or 50 feet wide. A map by the railway engineers made August 24,1907, shows a bar of coarse gravel and bowlders along the whole front of the glacier, with a width of 100 to 300 feet and with shallow water near the western bank. The main channel was then near the eastern bank of the river. The horizontal variation in position of the water margin on the eastern bank was 100 to 150 feet with the 24 foot seasonal rise and fall of the river level.
Beginning of Advance in 1909. The width of the river in front of Childs Glacier remained fairly constant from 1900 to 1908, showing variations in 1909, as indicated below.
After applying a correction for the seasonal rise and fall of the river, the decrease in width of the river from 1908 to 1909 shows an advance of the glacier of from 300 to 500 feet sometime during the summer of 1909, as indicated in the following table: